An integral aspect of the field of microbiology is the ability to positively identify microorganisms at the level of genus, species or serotype. Correct identification is not only an essential tool in the laboratory but plays a significant role in the control of microbial contamination in the processing of food stuffs, production of agricultural products and monitoring of environmental media such as ground water. Increasing stringency in regulations which apply to microbial contamination have resulted in a corresponding increase in industry resources which must be dedicated to contamination monitoring.
Of greatest concern is the detection and control of pathogenic microorganisms. Although a broad range of microorganisms have been classified as pathogenic, attention has primarily focused on a few bacterial groupings such as Escherichia, Salmonella, Listeria and Clostridia. Typically, pathogen identification has relied on methods for distinguishing phenotypic aspects such as growth or motility characteristics, and immunological and serological characteristics. Selective growth procedures and immunological methods are the traditional methods of choice for bacterial identification, and can be effective for the presumptive detection of a large number of species within a particular genus. However, these methods are time consuming, and are subject to error. Selective growth methods require culturing and subculturing in selective media, followed by subjective analysis by an experienced investigator. Immunological detection (e.g., ELISA) is more rapid and specific, however it still requires growth of a significant population of organisms and isolation of the relevant antigens. For these reasons interest has turned to detection of bacterial pathogens on the basis of nucleic acid sequence.
It is well known, for example, that nucleic acid sequences associated with the ribosomes of bacteria are often highly conserved across genera and are therefore useful for identification (Webster, U.S. Pat. No. 4,717,653 and U.S. Pat. No. 5,087,558; Enns, Russel K. Lab. Med., 19, 295, (1998); Mordarski, M. Soc. Appl. Bacteriol. Tech. Ser., 20 (Chem. Methods Bact. Syst.), 41, (1985)). Weisburg et al., (EP 51736) disclose a method for the detection and identification of pathogenic microorganisms involving the PCR amplification and labeling of a target nucleotide for hybridization to 16S rDNA of E. coli. and Lane et al., (WO 9015157) teach universal nucleic acid probes that hybridize to conserved regions of 23S or 16S rRNA of eubacteria.
Although bacterial ribosomal nucleic acids contain highly conserved sequences, they are not the only sources of base sequence conservation that is useful for microorganism identification. Wheatcroft et al., (CA 2055302) describe the selection of transposable elements, flanked by unique DNA sequences for the detection of various Rhizobium strains. Similarly Tommassen et al., (WO 9011370) disclose polynucleotide probes and methods for the identification and detection of gram-positive bacteria. The method of Tommassen et al., relies on probes corresponding to relatively short fragments of the outer membrane protein OmpA, known to be highly conserved throughout gram-positive genera. Atlas et al., (EP 517154) teach a nucleic acid hybridization method for the detection of Giardia sp. based on designing probes with sequences complementary to regions of the gene encoding the giardin protein. Webster, J. A., (U.S. Pat. No. 4717653) has expanded upon the use of rRNA in disclosing a method for the characterization of bacteria based on the comparison of the chromatographic pattern of restriction endonuclease-digested DNA from the unknown organism with equivalent chromatographic patterns of at least 2 known different organism species. The digested DNA has been hybridized or reassociated with ribosomal RNA information-containing nucleic acid from, or derived from a known probe organism. The method of Webster et al., effectively establishes a unique bacterial nucleic acid "fingerprint" corresponding to a particular bacterial genus against which unknown "fingerprints" are compared.
The methods described above are useful for the detection of bacteria but each relies upon knowledge of a gene, protein, or other specific sequence known a priori to be highly conserved throughout a specific bacterial group. An alternative method would involve a nontargeted analysis of bacterial genomic DNA for specific non-phenotypic genetic markers common to all species of that bacteria. For example, genetic markers based on single point mutations may be detected by differentiating DNA banding patterns from restriction enzyme analysis. As restriction enzymes cut DNA at specific sequences, a point mutation within this site results in the loss or gain of a recognition site, giving rise in that region to restriction fragments of different length. Mutations caused by the insertion, deletion or inversion of DNA stretches will also lead to a length variation of DNA restriction fragments. Genomic restriction fragments of different lengths between genotypes can be detected on Southern blots (Southern, E. M., J. Mol. Biol. 98, 503, (1975). The genomic DNA is typically digested with any restriction enzyme of choice, the fragments are electrophoretically separated, and then hybridized against a suitably labelled probe for detection. The sequence variation detected by this method is known as restriction length polymorphism or RFLP (Botstein et al. Am. J. Hum. Genet. 342, 314, (1980)). RFLP genetic markers are particularly useful in detecting genetic variation in phenotypically silent mutations and serve as highly accurate diagnostic tools.
Another method of identifying genetic polymorphic markers employs DNA amplification using short primers of arbitrary sequence. These primers have been termed `random amplified polymorphic DNA`, or "RAPD" primers, Williams et al., Nucl. Acids. Res., 18, 6531 (1990) and U.S. Pat. No. 5,126,239; (also EP 0 543 484 A2, WO 92/07095, WO 92/07948, WO 92/14844, and WO 92/03567). The RAPD method amplifies either double or single stranded nontargeted, arbitrary DNA sequences using standard amplification buffers, dATP, dCTP, dGTP and TTP and a thermostable DNA polymerase such as Taq. The nucleotide sequence of the primers is typically about 9 to 13 bases in length, between 50 and 80% G+C in composition and contains no palindromic sequences. RAPD detection of genetic polymorphisms represents an advance over RFLP in that it is less time consuming, more informative, and readily susceptible to automation. Because of its sensitivity for the detection of polymorphisms RAPD analysis and variations based on RAPD/PCR methods have become the methods of choice for analyzing genetic variation within species or closely related genera, both in the animal and plant kingdoms. For example, Landry et al., (Genome, 36, 580, (1993)) discuss the use of RAPD analysis to distinguish various species of minute parasitic wasps which are not morphologically distinct. Van Belkum et al., (Mol. Biochem Parasitol 61, 69, (1993)) teach the use of PCR-RAPD for the distinction of various species of Giardi.
In commonly assigned application U.S. Ser. No. 07/990,297, Applicants disclose a method of double-nested PCR which is used to detect the presence of a specific microbe. This disclosure first describes identifying a random unique segment of DNA for each individual microorganism which will be diagnostic for that microorganism. To identify and obtain this diagnostic nucleic acid segment a series of polymorphic markers is generated from each organism of interest using single primer RAPD analysis. The RAPD series from each organism is compared to similarly generated RAPD series for other organisms, and a RAPD marker unique to all members of the group is then selected. The unique marker is then isolated, amplified and sequenced. Outer primers and inner primers suitable for double-nested PCR of each marker may then be developed. These primers comprise sequence segments within the RAPD markers, wherein the inner set of primers will be complementary to the 3' ends of the target piece of nucleic acid. These nested primers may then be used for nested PCR amplification to definitely detect the presence of a specific microorganism.
In the present method Applicants have more particularly adapted and more fully described this RAPD methodology to identify a sequence, or marker; the presence of which will be diagnostic for all individuals of a genetically related population. The present method first involves a RAPD amplification of genomic DNA of a representative number of individuals within a specific genus, species or subspecies to produce a RAPD amplification product, termed the diagnostic fragment. This diagnostic fragment must be present in the RAPD profiles in over 90% of the individuals tested. Sequence information from the diagnostic fragment will then enable identification of the most suitable PCR primer binding sites within the diagnostic fragment to define a unique diagnostic marker. Primers flanking this marker will be useful to produce an amplification product in the genetically selected group, but will not produce any amplification product in individuals outside of that group.
An important aspect of the present invention is the identification of the most conserved primer binding sites within this diagnostic sequence, which is accomplished by first determining which individuals, in the genus or grouping to be detected, exhibit the most genetic variation within the diagnostic sequence. Screening this subpopulation of "most polymorphic" individuals using various primers generated from the diagnostic sequence will define the most highly conserved primer bindings sites within the diagnostic fragment. Primers directed toward these highly conserved primer binding sites are then useful for the detection of all members of the genus, based upon the ability of the selected primers to amplify the diagnostic marker present in that particular population. A "yes" or "no" answer can then be readily provided to the question of whether a microorganism is a member of the genetically related population. If DNA amplification occurs using these primers, the target is present and the identity is confirmed as "yes". If amplification does not occur, the answer is no; the microorganism is not a member of that genetically related population. The necessity of electrophoresis to determine the presence of a marker of any particular size is eliminated.
Applicants' method is distinctive in that to accomplish detection of a member of a group of organisms, the method first relies on determining the most conserved regions of a diagnostic fragment from a phenotypically uncharacterized segment of DNA common to all members of that group. One of skill in the art will recognize that conservation of sequence may represent both an ally and an enemy in the process of identification of the members of a particular genus. For example, many bacterial sequences are conserved across genera and hence would not be useful in the determination of species within a particular genus. It is precisely for that reason that methods heretofore elucidated in that art rely primarily on the analysis of sequences derived from proteins or genes known to be specific to a particular genus, i.e., ribosomal RNA or outer membrane proteins. Applicants' method departs from the art in that the conserved sequences of the instant method are not derived from a known gene, nor is the sequence associated with any known phenotypic characteristic. Further, Applicants' method is refined by the selection of the most conserved region of the diagnostic fragment by comparison with the genomic DNA of a subpopulation of individuals exhibiting the most genetic variation within the diagnostic fragment. Applicants' method presupposes that the regions of the diagnostic fragment most conserved within the polymorphic subpopulation will also be conserved within the larger population comprising all members of the genus. Applicants are unaware of any art teaching this supposition.
The process of the present invention has been enabled in the present disclosure by the elucidation of a diagnostic marker sequence which is useful in rapidly and definitively identifying bacteria from the genus Salmonella.